In Broadcast June 2012 - 30

WWW.INBROADCAST.COM | ISSUE 9 - JUNE 2012

InSOUND HD Voice Provides A Secret Weapon
Mobile phones have historically been overlooked by broadcasters as a practical broadcasting solution. With the introduction of the HD Voice technology, Glensound Electronics has launched their HD Voice Broadcasters’ mobile phone range to bring 7kHz audio bandwidth on mobile phones speciﬁcally designed for broadcast. HD Voice, coined by the telecom companies, is the commercial name for the G.722.2 AMRWideband Codec. This technology was developed to overcome the 300-3400Hz limitation traditionally used in telephony. HD Voice technology has an extended bandwidth of 50-7000Hz giving a superior natural sound quality and increased intelligibility. The frequency response is broadly similar to G.722 which is widely used in broadcasting. coding known as ‘Linear Predictive Coding’. The lower rate was introduced to reduce the congestion due to trafﬁc surges and increase the available bandwidth for further calls, thus improving revenue for the telecommunications companies. In 1997, an enhanced Full Rate codec was developed which incorporated both the Full Rate and Half Rate codecs and added a higher quality codec. AMR-NB (Adaptive Multi Rate Narrowband) used an ‘Algebraic Code Excited Linear Prediction’ algorithm (ACELP) and as before, used an 8kHz sampling frequency giving 3.4kHz bandwidth all operating within the existing GSM channels running at bit rates from 4.75 to 12.2 kbits/s. In an age of high deﬁnition, there became a need to raise the quality of voice calls that, up to this point, had made little progression in 50 years. As a response, AMR-WB, standing for Adaptive Multi Rate Wideband, was introduced and standardised as G.722.2. AMR-WB uses a similar speech coder to the ACELP used in AMRNB; the crucial difference being its use of a 16kHz sampling frequency enabling the bandwidth to be extended to 7kHz at the upper limit and 50Hz at the lower limit. The 300Hz lower limit found in AMR-NB, at best, causes audio to appear thin regardless of its upper

Paul Grant

Chief Engineer Paul Grant, Glensound Electronics, discusses how HD Voice on mobile phones will make outside broadcasts better, easier and more reliable

Glensound GS-MPI004HD

limit, whereas the 50Hz lower limit in AMR-WB improves the natural sound of the voice.

HD Voice
The AMR-WB codec bit rates run from 6.6kbit/s to 23.85kbits/s, the higher bit rates are not used within the mobile phone network but are used in other applications such as storage. The standard within mobile phone networks is 12.65 kbits/s, giving a quality similar to G.722 running at 56 bits/s, chosen to ﬁt within the 13kbits/s familiar to GSM allowing interworking with GSM networks. To cope with poor radio conditions, the network can temporarily reduce the bit rate; the 7kHz bandwidth is maintained but with reduced quality. MUSHRA is a scale used within the telecoms industry as a way of measuring the subjective quality of

audio. As depicted in Table 1, the horizontal axis shows the frequency response and the vertical axis shows the incremental rises in subjective audio quality. This demonstrates the superior subjective quality of HD Voice over voiceband at normal bit rates. These levels decrease proportionally with a reduction of bit rates in poor radio areas.

Brief History
Over the last 50 years, telephony has been limited to the 3.4kHz developed for early analogue telephone systems. Frequency division multiplexing was used to squeeze more voice channels into trunk circuits. The frequency of separation used in the carriers was 4KHz thus restricting the audio bandwidth. Digital telephony was later developed but had to integrate with the existing analogue technology. A sampling frequency of 8kHz was used, broadly matching the audio bandwidth of the existing analogue telephone systems. This value was also a multiple of 4kHz which reduced interference with the frequency division multiplex systems. When the mobile phone network emerged, it made sense, particularly with the limitations of the technology at the time, to integrate them with the existing technology and use the same sampling frequencies and audio bandwidth. The early GSM data channels available where either Full Rate (13kbits/s) or Half Rate (6.5kbits/s) and used a form of

Call Voice Connections
Glensound HD Voice calls connect over the 3G network in the same way as normal voice calls and are therefore given priority over packet data in order to maintain quality of service. Once a call is established, it should not be affected as network trafﬁc increases, conversely packet data may be restricted causing dropouts in codecs that use packet data.
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Table 1 - MUSHRA HD Voice Quality

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In Broadcast June 2012

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